FIG. 9 is a block diagram showing a conventional semiconductor apparatus provided with an ESD protective circuit.
An analog functional block 105a and a digital functional block 105d are formed in an internal circuit 105 formed on a semiconductor substrate. The analog functional block 105a and the digital functional block 105d are electrically connected to each other via an interface 111. An analog voltage AVcc as a power supply voltage and an analog ground AGND are electrically connected to the analog functional block 105a. A digital voltage DVcc as a power supply voltage and a digital ground DGND are electrically connected to the digital functional block 105d. 
An analog pad 109a is electrically connected to the analog functional block 105a via the ESD protective circuit 107a. The analog power supply voltage AVcc and the analog ground AGND are electrically connected to the ESD protective circuit 107a. 
A digital pad 109d is electrically connected to the digital functional block 105d via the ESD protective circuit 107d. The digital power supply voltage DVcc and the digital ground DGND are electrically connected to the ESD protective circuit 107d. 
The ESD protective circuits 107a and 107d are composed of, for example, metal oxide semiconductor field effect transistors (MOSFETs) and diffusion resistors (for more details, refer to Japanese Laid-open Patent Applications 8-37299, 8-236637, 8-288404, and 9-186296).
FIG. 10 is a circuit diagram showing an equivalent circuit of the ESD protective circuit 107a. 
The ESD protective circuit 107a is configured by a protective diode D1 formed by an N-channel type MOSFET, a protective diode D2 formed by an N-channel type MOSFET, and a diffusion resistor R.
The diffusion resistor R is provided on the signal line between a MOSFET that is a portion of the analog functional block 105a and the analog pad 109a. The source of the protective diode D1 is connected to the analog power supply voltage AVcc. The drain of the protective diode D1 and the drain of the protective diode D2 are connected to each other, and further connected to the signal line between a diffusion resistor 8b and the analog pad 109a. The gate electrode of the protective diode D1, the source of the protective diode D2, and the gate electrode of the protective diode D2 are grounded.
As the components of large scale integrated circuits (LSIs) become smaller, lightly doped drain (LDD) type MOSFETS, of which the ability to withstand ESD voltage is lower than that of single drain structured MOSFETs, are used more often than ever. Accordingly, it is getting more difficult to form protective components of the ESD protective circuit with only MOSFETs.
Further, since the integration scale of LSIs is getting greater, it is possible to form many system blocks on a single chip wherein many digital functional blocks and many analog functional blocks are disposed. In the case of a semiconductor apparatus provided with multiple digital functional blocks and multiple analog functional blocks, it is necessary for each functional block to have its own power supply in order to protect each functional block from the noise on common signal lines, for example, and to reduce power consumption of the entire LSI by using and managing multiple power supply voltages. Most of the multiple power supply voltages are generated external to the LSI and provided to the LSI through separate terminals of the LSI.
Further, in the case of a system LSI having an analog functional block, an exclusive power supply system is often used for the analog functional block, which requires a small-sized ESD protective circuit. Accordingly, it becomes obvious that the ESD withstanding voltage of such a system LSI becomes low.
Further, as showed in FIG. 9, in the case of an LSI having multiple power supply systems, if a large current needs to flow through the analog pad 109a, the electrostatic energy cannot be discharged by the ESD protective circuit 107a since the analog power supply AVcc and the analog ground AGND do not have enough capacity to let the current flow. The electrostatic energy may also be discharged through the digital power supply DVcc and/or the digital ground DGND. Thus, the internal circuit including the interface 111 electrically connecting the analog functional block and the digital functional block are damaged by the ESD.